The present invention generally relates to battery cells and methods for bilateral energy transfer therebetween and, more particularly, to an autonomous lithium-ion cell balancing system with integrated voltage monitoring and a method for autonomous cell balancing of multiple lithium-ion cells.
Spacecraft, such as satellites, require electric power to operate on-board equipment. Electrical power supply and energy storage subsystems that are part of the bus equipment of a spacecraft control power generated by solar paddles as well as power stored in batteries and supply electric power at a certain stable voltage at all times to the on-board equipment. Even a short period of malfunction in the power supply subsystem would disturb normal operation of the equipment of a spacecraft, which could result in a failure of the mission of the spacecraft. Since the power required by the on-board equipment tends to become higher, the mass and size of the electrical power supply subsystems tend to become greater too. The overall mass and the dimension of spacecraft are strictly limited due to a limited capacity of launch vehicles. Also the costs for launching a spacecraft increase with the mass and dimension of the spacecraft. Therefore, it is desirable to design electrical power supply and energy storage systems that are reduced in size and weight.
Due to the mass and cost advantage, the lithium-ion battery technology, which is already widely used for cellular phones and personal computers, is rapidly overtaking presently used NiH2 or Ni—Cd batteries as the standard for future energy storage systems for spacecraft. Lithium-ion batteries are able to store 3 to 4 times as much electric power as currently used batteries. Therefore, by using lithium-ion batteries instead of, for example, NiH2 batteries, the mass savings for a large spacecraft would average 150 kg, which equates to per launch savings of $10 million. However, prior art lithium-ion battery technology has the disadvantage that it requires complex and costly electronic control circuitry.
A prior art individual cell charge control system 10 for cell balancing is shown in a simplified block diagram in FIG. 1. Such individual cell charge control system 10 utilizes cell charge regulators 13 such as flyback regulators or forward converters that have a feed back of the cell voltage to force all cells 14 of a battery 15 to regulate to the same voltage. As shown in FIG. 1, a single differential amplifier 11 senses a battery voltage and generates an error signal 12. The error signal 12 is used as the reference for all cell charge regulators 13. All regulators 13 operate in current limit until the battery voltage is equal to the error voltage and consequently, all cells 14 are charged to the same voltage. One major disadvantage of this prior art approach, as illustrated in FIG. 1, is that each cell 14 needs to be regulated individually. Further, fly back converter or forward converter are relatively complex and require precise control of the duty cycle to maintain cell balance.
A further prior art system 20 for balancing the cell voltages of the cells 21 of a battery 22 is the use of resistors 23 connected by relays 24 across each cell 21 combined with cell voltage monitoring 25 and either ground control or computer control 26, as shown in an simplified block diagram in FIG. 2. A relay 24 is temporarily closed to discharge a cell 21 having a higher cell voltage than the other cells 21 of the battery 22. When a cell is adequately balanced, the relay 24 is opened. The prior art approach to balance the cell voltage of the cells 21 of a battery 22, as shown in FIG. 2, requires ground control or computer control, for example by a spacecraft computer 26. The resistors 23 dissipate a significant amount of power. Further, high precision cell monitoring provided by a battery charge voltage monitor 25 is required. Still further, to reach full redundancy, two relays 24 and two resistors 23 as well as two battery charge voltage monitors 25 are necessary. There has, therefore, arisen a need for a system that is able to balance and monitor the energy levels of lithium-ion battery cells that is reliable and of relatively low cost. There has further arisen a need to provide an energy monitor/control system for lithium-ion batteries that allows improved monitoring accuracy resulting in higher storage capability of the battery cells and therefore, reduces the energy system mass of a spacecraft as well as the per launch costs. There has also arisen a need to provide a power balancing system for lithium-ion cells with an improved reliability by reducing the overall system parts count. There has further arisen a need to provide a power balancing system for multiple lithium-ion cells that allows to balance groups of cells rather than the whole battery and that allows to disable balancing for bypassed cells. There has still further arisen a need to provide a cell balancing system that allows to terminate drive to failed converter cells and that enables cell voltage monitoring of the battery cells on the secondary side.
As can be seen, there is a need for an autonomous cell balancing system that is able to balance the energy level of lithium-ion battery cells continuously and reliably. Also, there is a need for a lithium-ion cell balancing system that allows increased energy storage by reduced overall system mass and therefore, reduces the cost per launch of a spacecraft. Moreover, there is a need for an autonomous cell balancing system that eliminates the need of a separate cell voltage monitor.